1. Field of the Invention
The invention relates to recovery of ammonia in gas purged out from a synthesis loop in an ammonia producing factory.
The invention is specifically directed to adsorption of the ammonia in purge gas in a packed bed and returning the adsorbed ammonia to the synthesis loop.
2. Description of Related Art
The traditional purge gas absorption is carried out as absorption in water in counter-current followed by recovery by distillation. This is simple and reliable but uneconomic in terms of investment and also in terms of energy consumption since medium pressure steam is needed for the distillation, cleaned washing water has to be pumped back to the absorption tower and some of the liquid streams have to be cooled down by cooling water.
Alternatively, the purge gas could be routed through a molecular sieve or another porous ammonia absorbing bulk material.
Isalki discloses a process for ammonia adsorption from a purge gas in U.S. Pat. No. 4,266,957. This process involves regeneration of the adsorbing agent at a pressure very near to atmospheric pressure by flushing out the adsorbed ammonia with hydrogen from the cleaned purge gas.
In U.S. Pat. No. 4,077,780, Doshi describes another process for adsorption of ammonia and methane in a purge gas from an ammonia synthesis loop. Here four adsorbers are involved and the regeneration of adsorbing agent takes place at approximately atmospheric pressure in this pressures wing adsorbing process. Some of the cleaned purge gas is used for the regeneration, and the effluent gas from the regeneration is sent to a waste manifold.
The invention relates to ammonia synthesis loops, in which some of the gases do not react and would accumulate if they were not purged out. However, these purge gas streams contain some amounts of valuable product ammonia.
This ammonia is then recovered by distillation of the washing water or flushing out from the adsorbing agent at low pressure.
The mentioned processes require investments for distillation systems, washing water circulation systems, including pumps and heat exchangers, valves for depressurisation and returning to high pressure synthesis loop, besides cost of energy for pumps, compressors and steam for distillation.
The object of the invention is to provide an improved ammonia synthesis process by recovering the ammonia in the purge gas with an adsorption agent operating at the full synthesis loop pressure. The adsorption agent is chosen in such a way that the ammonia can be removed again by passing a gas containing hydrogen and nitrogen through it at the same elevated pressure as the loop pressure. This enables the adsorption agent to be regenerated by fresh synthesis gas coming from a synthesis gas compressor before this gas enters the synthesis loop.
Accordingly, the present invention provides a process for removal and subsequent recovery of ammonia from a purge gas stream comprising ammonia, hydrogen, nitrogen, argon, helium and methane in an ammonia synthesis loop comprising the steps of introducing the purge gas stream into a first ammonia adsorber comprising a selective ammonia adsorbing agent, adsorbing ammonia in the purge gas on the ammonia adsorbing agent at the same pressure as the pressure of the synthesis loop, withdrawing an ammonia free purge gas from the first ammonia adsorber, introducing a make-up gas stream comprising hydrogen, nitrogen and traces of at least one of argon, helium and methane to a second ammonia adsorber comprising a selective ammonia adsorbing agent saturated with ammonia, removing ammonia from the ammonia adsorbing agent by the make-up gas at the same pressure as the pressure in the synthesis loop, withdrawing an ammonia containing effluent from the second ammonia adsorber and introducing the effluent to the ammonia synthesis loop at the normal make-up gas addition point, and introducing after a pre-set period of time the purge gas to the second ammonia adsorber and simultaneously introducing the ammonia free make-up gas to the first ammonia adsorber as in the above steps.
This is a method for cheap, effective regain of ammonia from a purge gas stream without high investments and costs for distillation towers, washing water circulation pumps, compressors, heat exchangers, steam and several control valves.